law enforcement and intelligence agencies are racing to learn. was there foreign help or training? older brother tamerlan spent six months overseas last year. he went to chechnya, areas where islamic extremists and violent chechen separatists are known to operate. >> he was away for six months. was that long enough to have done what? >> that's long enough to have done a lot of things. if you're motivated, have the connections, there's plenty of time there to be initiated into a group, receive training and potentially a lot of training. >> reporter: robert mcfadden is a former agent in the n.c.i.s., the naval criminal investigative service. his 30-plus years experience in counterterrorism includes work on the al qaeda bombing of the u.s.s. coal in 2000. >> something just blew up. reporter: mcfadden believes the number one priority of investigators is to be sure there are no accomplices still at large. >> are there other events coming? what other events are planned and doing everything possible to rule out that there could be other terrorist acts in place and do everything possible t